The Mechanism of Chromatographic Retention - Chiral Chromatography > Chiral Polysiloxane Stationary Phases > Page 75

One or two of the primary hydroxyl groups (position 6) are those normally used to link the cyclodextrin to the silica surface. The secondary hydroxyl groups (positions 2 and 3) can be selectively derivatized, first in position 2 and then in position 3. A number of derivatives of the cyclodextrins have been synthesized to provide specific types of interaction to increase their chiral selectivity.

X-ray data has indicated that the b and g structures are quite rigid whereas the a structure appears to be somewhat flexible. Thus solute molecules, if spatially suitable, can be included in cyclodextrin cavity and interact by dispersive, polar of ionic forces with any neighboring groups to which they are appropriately close. The inclusion of a solute by the cyclodextrin structure is depicted in figure 37.

Thus, if there is spatial differences between a pair of isomers, then this can introduce some interactive selectivity. This mechanism will be exhibited as another type of entropic contribution to the standard free energy of distribution which will also induce an attending enthalpic contribution. The concept, depicted in figure 37, is a grossly over simplified impression of the inclusion phenomena.

Courtesy of ASTEC Inc.

Figure 38. Diagram Depicting Solute Inclusion in the Cyclodextrin Structure